Very nice stay in a small apartment with a bed for two, small kitchen, comfy couch, nice bathroom, working AC and beautiful view. On the 6th floor is a pool, gym and laundry machines (around 60 THB per laundry).
To check in you need to get in the door which is locked and guarded by a security guard. The best thing is to just sneak in when someone else opens the door as the guard doesn't care, then finding the mailbox was quite easy.
From the apartment one can walk to the sky train station Whutakat in about 5 min, from where one can get a train ticket for 59 THB to the center (CEN).
Overall we had a nice experience for the 18€/night and can recommend this stay.